Electric signal transmission system



Dec. 20, 1949* l V P. FKM. GLOESS 9 ELECTRIC SIGNAL TRANSMISSION SYSTEM Filed Oct. 14, 1946 g"?- "8 "7 4 ssuzc'ron 9 l fl l f 5 f6 GENERATOR Low PASS I75 MODULATOR D FILTER 1 v NEG. FEEDBACK TO MODULATING SIGNAL PRODUCE LINEARITY GENERATOR Fig I SYNCHRONIZING PULSE 26 PULSE GENERATOR 27 29 GENERATOR DEMODULATOR LOWPASS SELECTOR @Q IH L DEMODULATOR MIXER 2| LOWPASS SELECTOR Modulator 2% 24mm .6

25 23 \k 24 1 PULSE SELECTOR a uEeJ-EEoeAcK DISTRIBUTOR F g 2 ormoouw'cm UNITED STATES PATENT OFFICE 2,491,969 ELECTRIC SIGNAL TRANSMISSION SYSTEM Paul Francois Marie GloessfPaiis, France, as-

signor to Socrete francaise Sadir Carpenti'er, Paris, France, a corporation of France Application October 14, 1946, Serial No. 703,220 In France October 25, 1945 7 Claims. 332-9) y invention a es to systems for transmitnegative feedback circuit across a modulator can ting electric signals wherein the signals to be be obtainedlonly by increasing the frequency of transmitted are used for modifying the particular recurrence of the carrier pulses, i. e. by using for features such as the amplitude, duration and relatransmitting purposes a too much extended high tive spacing of recurrent series of impulses of a frequency spectrum. higher frequency. It is known that, in such pulse These drawbacks are overcome according to modulation systems the frequency of the pulses certain features of my invention, while retaining should be at least twice that of the maximum however the advantages of a negative feedback frequency of the modulating signals and the circuit associated with the modulator. This is ratio generally adopted for these two frequencies 10 obtained by the process of modulating, by the is 2.25 so as to provide the marginal parts for signal to be transmitted, a series of impulses the the required filtration without any waste of frefrequency of which is an integral multiple of the quency channels. frequency of the modulated pulses which will In such systems however, a particular difficulty finally be sent on to the transmission system and arises for obtaining a transmission of high grade by picking up modulated pulses at the output, a

as soon as it is desired to establish a pulse moduseries of pulses which are repeated at the desired lator with a very small rate of linear distortion. frequency of transmission. In practice, with any known arrangement it is Atransmission system for modulating impulses, possible to obtain distortion rates of the magniembodying features of my invention, includes tude of 5% for a mean depth of modulation therefore (1) a generator of carrier pulses which whereas it may sometimes be desired to obtain are repeated at a frequency F=n F, F being less than 1%. i the frequency of the modulated pulses which it It is then possible to make use of the advanis desired to transmit in the system; (2) a modutages provided by a negative feedback device aslator provided with a negative feedback circuit sociated with the modulator. It is known that as described which modulates the pulses of a freif the low frequency gain in the feedback loop quency F with the modulating low frequency obtained is equal to as, ,a being the gain of the signal; and (3) a pulse selector for picking up direct circuit and [3 that of the negative feedback from the series of pulses thus modulated, with circuit, the rate of non-linear distortion obtained a reduced rate of distortion at a frequency F, is divided by the factor (l mp). a series of modulated pulses which are repeated However for such an arrangement, it is necesat a frequency F and (4) means for sending these sary to provide in the direct transmission path pulses at a recurrence frequency F into the transbeyond the modulator, an auxiliary demodulator mission circuit. of a design similar to that of an ordinary receiver Such an arrangement is of advantageous appliset demodulator, and a low pass filter eliminating cation according to a further feature of my inundesired frequencies such for instance as the vention, to the case of multiplex communications.

frequency of the difference between the recur-- Asa matter of fact, in said case, the periodicity rence frequency of the carrier pulses and the of the pulses at a higher recurrence, as used for highest frequency of the modulating signal, 1. e. modulation, equal to that of the pulses of the frequencies of the same magnitude as the higher complex signal constituted by the combination frequencies of said modulating signal, for the of the impulses of all the channels. usual above-mentioned ratio of the magnitude These features and also other features and the of 2.25. manner of embodying same, will be disclosed The presence and dimensioning of the low pass with further detail in the following description filter in the feedback loop produces a delay of given by wayof example and with reference to transmission which is comparatively important accompanying drawings. In said drawings: and the amplitude of the phase shifts for the Fig. lis a diagram illustrating a portion of higher frequencies of the band to be transmitted, a simplex transmitter for a pulse modulation which may reach in fact phase opposition, limits system; i i the gain of the loop to a value less than 1, which 5 Fig. 2 is a diagram of a portion of a multiplex leadsto a particularly low improvement of the transmitter for a similar transmlssion.

rate of distortion for lower values of said band Fig. 3 shows diagrams of pulses used for exof frequencies and, actually, to a worse distortion plaining the-operation. ion-the higherfrequenciesto be transmitted. II! 1 1, su o t a h m x um te h It seems therefore that the advantage of a frequency to be transmitted is for instance 1.0,-

000 cycles per second, the frequency of the pulses to be considered will be of the magnitude of 25,000 per second. Now if a pulse generator were directly used, operating at this frequency and if it were attempted to apply to the modulator following this generator a negative feedback circuit, the filter of thefeedback. loop should cut off the frequencies comprised between 1 and 1.5 times the maximum telephonic frequency, i. e.

the frequencies above a value comprised between 10,000 and 15,000 cycles In order to avoid the per second, and above.

according to the invention, a pulsefgeneratorthe an integra muiu ie at:

frequency of which is limitations lace to small difference in frequencies thereis provided that of the pulses which are finally transmitted v after modulation. In enerator i which is seven times to be transmitted; such a series of pulses may be similar to that shown at El 5i on the curve of.,' Fig,;' s.

Thesepulses are applied tothe modulator 3 together with the modulating, signal generated This modulator is connected by the'source' 2. to a negative feedback circuit arrangement throu h the agency of an auxiliary demodulator 5 of adesign similar to that of the usual receiv ing 'demodulatorl' Said demodulator ,is followed by a, low bandfilter i5 and a negative feedback circuit which mayinclude an attenuator, a phase shifter or a simple connection according to' 'arrang'ements well known in the art.

The pulses thus pulses. The'connection 8 shown in dotted lines supplies these pilot pulses to the circuit 4 that is provided witha circuit such as that disclosed in l copendin'g applicationSerial No. 761;029, filed by Paul Gloess and Gaston Potier on July 15, 1947; for Pulse frequency division and assignee'.

The frequency of the pulses applied to the modulator 3 in the above described system is F'=-'"IF, .F being the frequency pulses at the output.

oi the modulated Supposing F'is equal 'to 25,000 persecond, it is apparent that F'- 1 75,000 l pen'sec'ond. Thus the lower stray frequency to be eliminated in 165,000 per secondgii e. 16.5 times the value of f, the maximum telephonic frequency to be transmitted, instead of 1.5 times as would be thecase if the same pulse recurrence were used in the generator. I and in the output .circuit. The cutoff frequency of the filter 6 may thus be increased by eleventimes, its proportionally as well as the phase shift for the same frequency to be transmitted. It is consequentlypossible to'use a much higher degree of negative feedback, which produces a practically complete elimination of the non-linear distortions affecting the modulation.

A "correct dimensioning according to the means used generally in the art, for the circuit elements in es e iiir .i s ll a sl e i l i iree l the diagram'of 1 {the is chosen with a recurrence of pulses the recurrence of the, pulses modulated are transmittedto a circuitf l in which is produced a demultiplica to those used intelevision assigned re this j transmission time is reduced [75 of a modulator :thus c ,rangement may be arranged tion , signal including all y I together with'the pilot channel}. e.' in the case putof the modulator ll..

in an improvement by more than ten times of the linearity of the modulation obtained. The rate of distortion is reduced to 0.5%.

It is obviously supposed, in these estimations, that the auxiliary demodulator 5 does not introduce any substantial distortion, which result may be easilyreached in practice...

In the case of demodulators producinga predetermined distortion, the negative feedback arto correct the distorin the whole modulator-demodulator unit. It'is thus possible'to retain the advantage of a negligiblerate of distortion for the whole of the transmission, providing however that the desedilb'oth for transmission and for receptionshow asufficient constancy in their distortion, characteristics.

Ii'i'th'e case of a multiplex pulse system, several channels; such as telephone channels are transmitted simultaneously by,using several series of modulated pulses, interlaced. in. time, as ,showmin the diagrarnyc of Fig,s'wah a ,em15 ementar' seriesof unniodulated pulses GZ,..BL' a ing as pilot pulses for. sefparati g',, at the ,receiver the pulses belonging to ,e'ach channel For' applying the present. inuentipnft such. a range'ments, the periodicity of the pulses use modulation should bean integral multiple final periodicity of the pulses, of the, comp the transmission cha of a c annel y m-2.

F=K(n+1)F K being any suitable integer.

interval indispensable for tion operation.

In particular, it may be of advantage to take for k a value'l, i. e.

to adopt equal va ues for" the recurrence frequency of the pulses used Efojr' modulation (Fig. 3 curve 1?) complex signal (Fig. '3 curve e).

Fig. 2 illustrates diagrammatically an example of application of this invention andforfthatof the M i I t to a six channeli multiplex system, the corresponding pulse'dia grams being shown on curvesd to e in Fig. 3.-

The pulse generator i0 produces series of pulses of equal amplitudesand equal spacings 51- 41 5L as illustrated bythe curve a. It' ieeds the synchronizing pulse generator '25. .;with' pulses of high' frequency; pulses to the difierent modulators H, l2 and-l3 together with a pulse selector and distributor- 28.

Each modulator Ii, l2'and i3 includihg auxil iary means'such as Iamplifying and amplitude limiting devices, is suppliedby signalsources of The generator Iii I also feeds audio'frequencyfromi lines ill, 32 and 33,-res'pec tivelyf. The modulator. .l l is followedby. an.auxil..

iary demodulator I! followed inuturnibyaafllow pass filter i8. The output of. the. filter .iillis ap.-, plied by the negative feedback circuit [9 on the in;- As disclosed the ,high frequency of the pulses used for. modulationpur. poses allows the use of a filter the cut-off f'requency of which is high-with reference to the highest desired telephonic frequency whereby it is{ possibleio ake e of, n new mana er cuit."

The modulators l y k by similar demodulatorf'filterand 'feedba'ckpir:

2 and 13 are followed in'turn selectlngcircuit, l4, I5, lB-wherein the pulses m smndinsltceachchan are .dis r minated by m ans of c r nts] fromalcircuit 28 wh h may consist of a pulse selectorand distributor of any well known type; it may in fact be virtual i. e. incorporated iIlJiBEtCh of the selecting circuits l4, l and IS. The outcoming signals from the selectors have then a, recurrence frequency F as shownfor instance onthe curves 12,0 and d for the two first and the last modulator of the system with the corresponding shifts.

The synchronising pulses or pilot pulses at the output of the generator 26 are also demultiplied inside the circuit 21 in a manner similar to the pulses modulated by the signals.

All the pulses passing out of the selectors M to 21 are then transmitted to a mixing circuit 29; the shape of a composite signal appearing at 30 at the output terminals of said mixer, may be such as shown on the curve e of Fig. 3, the pilot pulses 6|, 6| being of same polarity but of a greater amplitude than the modulated pulses 52, 53 of the different channels although any other distinguishing feature such as a reversal of polarity may be used for this purpose. This composite signal may obviously be used as such for the transmission or for modulating suitable output stages.

What I claim is:

l. A method of transmitting modulated pulses o comprising generating pulses having a frequency that is an integral multiple of the frequency of the pulses to be transmitted, feeding said pulses to a modulator for modulating said pulses in accordance with signals to be transmitted, feeding part of the output of said modulator to a demodulator for demodulating said part of the modulator output, feeding the output of said demodulator through a negative feedback network to the input of said modulator to improve the linearity of said modulator, and selecting pulses from the other part of said modulator output corresponding to a submultiple of said first mentioned frequency, for transmission.

2. Apparatus for the transmission of modulated pulses comprising a generator for generating pulses of a frequency F" that is equal to n times F, F being the frequency to be transmitted, a modulator for modulating said pulses in accordance with signals to be transmitted, a demodulator for demodulating part of the output of said modulator, a low pass filter for passing the signal frequencies from said modulator, a negative feedback network connected between the output of said low pass filter and an input of said modulator for feeding said signals from said low pass filter to said modulator to improve the linearity of said modulator and means for periodically selecting a pulse from the frequency F of said modulated pulses such that the repetition rate of the selected pulses corresponds to the frequency F to be transmitted.

3. A multiplex transmission system for transmitting modulated pulses comprising a pulse generator for generating pulses of a frequency F, a plurality of modulators connected to the output of said pulse generator, a negative feedback network connected to each of said modulators for improving the linearity thereof, each of said feedback networks including a low pass filter having a cut-off frequency just below the frequency F whereby the degree of feedback in said feedback networks may be substantially increased and non-linear distortion in said modulators practically completely eliminated, a plurality of selectors, connections for applying the output of each of said modulators, to a selected one of said "selectors, means connected, between said pulse generator and said selectors for timing said selectors, and a mixerl'connectedltosaid selectors for interlacing the outputs of said selectors into a composite signal to be transmitted.

l; A multiplex transmission system for transmitting pulses "comprising a pulse generator for generating pulses, a plurality of modulators connected to said pulse generator, a negative feedback network for each of said modulators for improving the linearity thereof, each of said feedback networks including a low pass filter having a cut-off frequency just below the frequency whereby the degree of feedback in said feedback networks may be substantially increased and nonlinear distortion in said modulators practically completely eliminated, a mixer circuit, and means for selecting pulses from the outputs of said modulators in sequence and supplying said pulses in said sequence to said mixer circuit.

5. A multiplex transmission system as set forth in claim 4 further comprising a synchronizing pulse generator and connections for connecting said synchronizing pulse generator to said means for selecting pulses whereby said means selects synchronizing pulses and inserts said synchronizing pulses into the pulse sequence.

6. Apparatus for the transmission of modulated pulses comprising a generator for generating pulses of a frequency F that is an integral multiple of the frequency to be transmitted, a modulator for modulating said pulses in accordance with signals to be transmitted, a demodulator for demodulating part of the output of said modulator, a low pass filter for eliminating said frequency F, a negative feedback network connected between the out-put of said low pass filter and an in-put of said modulator for feeding the out-put from said low pass filter to said modulator to improve the linearity of said modulator, the cut-off frequency of said low pass filter be ing increased substantially up to said frequency F so that the transmission time and phase shift thereof is reduced whereby the degree of negative feedback in said network may be substantially increased and non-linear distortion in said modulator practically completely eliminated, and selecting means for selecting from said modulated pulses a series of pulses having a frequency that is a submultiple of the frequency F for transmission.

7. Apparatus for the transmission of modulated pulses comprising a generator for generating pulses of a frequency F that is equal to n times F, F being the frequency to be transmitted, a modulator for modulating the pulses of frequency F in accordance with signals to be transmitted, a demodulator for demodulating part of the output of said modulator, a low pass filter for eliminating the frequencies including the difference between the frequency F and the highest frequency of the modulating signal, a negative feedback network connected between the out-put of said low pass filter and an in-put of said modulator for feeding the out-put from said low pass filter to said modulator to improve the linearity of said modulator and means for selecting from said modulated pulses a series of pulses having a frequency F for transmission.

PAUL FRANCOIS MARIE GLOESB.

(References on following page) REFERENCES CITED Number Thehfollowing references are of record in the file of thls 'pement: 2,420,374

UNITED STATES PATENTS 5 Number Name Date 2,224,580 Wise Dec. 10, 1940 Number 2,314,707 Katzin Mar. 13, 1943 510,881

Name Date Earp Sept. 12, 1944 Baur Apr. 8, 1947 Houghton May 13, 1947 FOREIGN PATENTS Country Date Great Britain Aug. 8, 1939 

